Several technologies are available for the reduction of nitrogen oxides (NOx) from the exhaust of automobile engines. Three Way Catalysts (TWC) are designed to remove NOx from the exhaust of vehicles equipped with a gasoline engine. On the three-way catalyst, the nitrogen oxides react with the unburned hydrocarbons or CO in such a way that the oxygen of NOx is consumed for the oxidation of the unburned hydrocarbons or CO yielding nitrogen, carbon dioxide and water. Three-way catalysts cannot be used for the treatment of exhaust from lean burn and diesel engines because of the low NOx conversion in the presence of oxygen.
For diesel engines, there are two types of technologies for the reduction of NOx. The first is NOx storage and reduction, which requires alternating lean and rich operating periods of the engine. During the lean period, the nitrogen oxides will be adsorbed in the form of nitrates. During a rich period of operation, the nitrates are reduced to nitrogen by reaction with the hydrocarbons contained in the exhaust.
A second type of technology for the reduction of NOx in diesel engines involves Selective Catalytic Reduction (SCR) of NOx. A first type of SCR involves hydrocarbon SCR (HC SCR), which involves the use of a hydrocarbon such as diesel fuel as a reducing agent to reduce NOx in the diesel engine exhaust gas stream. However, the applicability of HC SCR for diesel engines does not appear to be viable because most of catalysts suitable for HC-SCR show a very narrow temperature window where a useful NOx reduction can be obtained. See, for example, Ishihara et al., Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997, in which conversion of NOx using Cu-SAPO-34 and hydrocarbons was observed to be less than 70% at about 400° C. and less than about 20% at 200° C.
A second type of SCR involves ammonia SCR. Selective Catalytic Reduction, using ammonia or ammonia precursor as reducing agent is believed to be the most viable technique for the removal of nitrogen oxides from the exhaust of diesel vehicles. In typical exhaust, the nitrogen oxides are mainly composed of NO (>90%), so the SCR catalyst favors the conversion of NO and NH3 into nitrogen and water. Two major challenges in developing catalysts for the automotive application of the ammonia SCR process are to provide a wide operating window for SCR activity, including low temperatures of from 200° C. and higher and improvement of the catalyst's hydrothermal stability for temperatures above 500° C. As used herein hydrothermal stability refers to retention of a material's capability to catalyze the SCR of NOx, with a preference for the retention to be at least 85% of the material's NOx conversion ability prior to hydrothermal aging.
The emissions from vehicles are measured using standardized engine or vehicle test cycles, in which speed and load are varied to simulate actual driving conditions. The ECE test cycle, which is also referred to as UDC, represents urban driving under low speed and load, and the Extra Urban Driving Cycle (EUDC) involves higher speeds. Most test cycles include a cold-start portion. For example, the Euro 3 test cycle includes the ECE+EUDC cycles, and includes evaluation of emissions when the catalyst at temperatures as low as 150° C. for a significant portion of the drive cycle. Thus, low temperature NOx conversion is of great interest.
Zeolites are aluminosilicate crystalline materials having rather uniform pore sizes which, depending upon the type of zeolite and the type and amount of cations included in the zeolite lattice, typically range from about 3 to 10 Angstroms in diameter. Both synthetic and natural zeolites and their use in promoting certain reactions, including the selective reduction of nitrogen oxides with ammonia in the presence of oxygen, are well known in the art.
Metal-promoted zeolite catalysts including, among others, iron-promoted and copper-promoted zeolite catalysts, where, for instance, the metal is introduced via ion-exchange, for the selective catalytic reduction of nitrogen oxides with ammonia are known. Iron-promoted zeolite beta has been an effective catalyst for the selective reduction of nitrogen oxides with ammonia. Unfortunately, it has been found that under harsh hydrothermal conditions, such as reduction of NOx from gas exhaust at temperatures exceeding 500° C., the activity of many metal-promoted zeolites, such as Cu and Fe versions of ZSM-5 and Beta, begins to decline. This decline in activity is believed to be due to destabilization of the zeolite such as by dealumination and consequent loss of metal-containing catalytic sites within the zeolite.
To maintain the overall activity of NOx reduction, increased levels of the washcoat loading of the iron-promoted zeolite catalyst must be provided. As the levels of the zeolite catalyst are increased to provide adequate NOx removal, there is an obvious reduction in the cost efficiency of the process for NOx removal as the costs of the catalyst rise.
Due to the considerations discussed above, there is a desire to prepare materials which offer improved low temperature SCR activity and/or improved hydrothermal durability over existing zeolitic materials, for example, catalyst materials which are stable at temperatures up to at least about 650° C. and higher, for example in the range of about 700° C. to about 800° C. and up to about 900° C. Moreover, since diesel engines operate under transient conditions, there is a desire to provide materials that exhibit high performance over a wide temperature range, from as low as 200° C. up to about 450° C. See Klingstedt et al., “Improved Catalytic Low-Temperature NOx Removal,” ACCOUNTS OF CHEMICAL RESEARCH/VOL. 39, NO. 4, 2006. Thus, while existing technologies provide high temperature performance, there is a need for materials can offer low temperature performance in predominantly NO feeds combined with hydrothermal stability. Low temperature performance is important for cold start and low engine load conditions.